Projection lens and system

ABSTRACT

Projection lenses and projection lens systems are telecentric between an illumination subsystem and a set of imagers. The lenses and systems can exhibit color fringing correction, uniform imager illumination, athermalization, and component articulation for improved imaging. The lenses and systems may be employed in display apparatuses, such as folded display apparatuses that have decreased footprint size, but long effective projection lengths.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of co-pending U.S. applicationSer. No. 09/177,931, filed Oct. 23, 1998, which is incorporated hereinin its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to projection lenses and projectionsystems, and, more particularly, to projection lenses and systems thatprovide improved use of the total light energy emitted by anillumination subsystem.

[0004] 2. Description of the Related Art

[0005] Light projection is used to display images on large surfaces,such as large computer displays or television screens. In frontprojection systems, an image beam is projected from an image source ontothe front side of a reflection-type, angle transforming screen, whichreflects the light toward a viewer positioned in front of the screen. Inrear projection systems, the image beam is projected onto the rear sideof a transmission-type, angle transforming screen and transmitted towarda viewer located in front of the screen.

[0006] In single exit pupil projection systems, three primary colorimages are projected through the same lens to form a full color image.These systems avoid color shift in the projected image and color mixingor combining need not be performed by their screen as in a three lenssystem. Single exit pupil systems may be either of the transmissivevariety or of the reflective variety. Additional information aboutprojection lenses and systems can be found in U.S. Pat. No. 5,218,480,issued to Moskovitch, entitled “Retrofocus Wide Angle Lenses,”incorporated by reference herein in its entirety.

[0007] Several considerations stand out for such projection systems. Oneitem is the efficient use of the light energy output of an illuminationsubsystem in a projection system. Matching the illumination subsystemwith imagers (e.g., a liquid crystal display (LCD) or spatial lightmodulator (SLM)) in the projection system to obtain a bright, uniformlyilluminated image is important. Etendue considerations have not beenparticularly emphasized in previous projection system designs. Examplesof the type of light sources in illumination subsystems, amongst others,for which efficiency can matter include metal-halide lamps and thosedescribed in U.S. Pat. Nos. 5,404,076 and 5,606,220, issued to Dolan etal., entitled “Lamp Including Sulfur” and “Visible Lamp IncludingSelenium or Sulfur,” respectively, and in U.S. Pat. No. Re. 34,492,issued to Roberts, entitled “Combination Lamp and Integrating Sphere ForEfficiently Coupling Radiant Energy From A Gas Discharge Into ALightguide.” U.S. Pat. Nos. 5,404,076, 5,606,220, and Re. 34,492 areincorporated by reference herein in their entirety. Other examplesinclude lamps described in PCT Pat. application No. PCT/US97/10490, byMacLennan et al., published as WO 97/45858 on Dec. 4, 1997, alsoincorporated by reference herein in its entirety.

[0008] Another consideration is system size. For rear projection andcomputer screen applications, a small overall package size is desirableexcept perhaps for the screen. The physical size of individualcomponents, such as lenses, filters, stops, etc., should be maderelatively small while a large image size should be produced. Although asystem may be small in size, however, its compactness may notnecessarily be optimized. For instance, in projection systems employingthree LCD imagers, one for each primary color, the distance between theprojection lens and the imagers may have to be increased to accommodatefield lenses required to better match the illumination subsystem and theimagers.

[0009] In some previous projection lenses, the filtering of image orimager illumination light has been of concern. A filter could be placed,for example, within an aperture stop of a projection lens. However,aperture stops have previously been disadvantageously positioned withinthe physical confines of one of the lenses or other elements making upthe projection lens.

[0010] Thermal effects have been a concern when polymer materials,despite their generally good optical properties, are used to constructindividual lens elements in projection lens systems. Aspheres, althoughuseful in limiting lens aberrations and in reducing lens size, canreveal detrimental thermal effects with high power light when positivelypowered optical elements are constructed of these materials. Acrylicmaterials, for example, present a relatively large change in refractiveindex with temperature. A lens fashioned out of acrylic can, therefore,display an internal temperature change or gradient. A correspondingoptical power change can result with high powered light, leading toperformance deficiencies.

[0011] Other considerations in projection systems include the effects ofdispersion in optical elements and manufacturing tolerances. Dispersioneffects frequently appear in optical systems in which all three primarycolors are transmitted through the same optical elements. Manufacturingtolerances can impact parts interchangeability. Manufacturing tolerancesmay result in performance variations that need to be addressed byappropriate means to ensure that production model projection lenses andsystems will demonstrate similar performance.

[0012] The present invention is directed to improving projection lensesand systems. The present invention is also directed to overcoming orreducing one or more of the problems and deficiencies set forth above orother problems and deficiencies.

BRIEF SUMMARY OF THE INVENTION

[0013] In general, in one aspect, embodiments of the invention feature aprojection lens system, that includes an illumination subsystemincluding a light source and a lightguide adapted to direct illuminationlight from the light source. The projection lens system also includes aprojection lens adapted to receive the illumination light from thelightguide and to direct image light derived from the illuminationlight.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the drawings in which:

[0015]FIG. 1 is a perspective view of a projection lens system inaccordance with a first embodiment of the invention.

[0016]FIG. 2 is a top view of the projection lens system in FIG. 1.

[0017]FIG. 3 is a more detailed top view of the projection lens systemin FIG. 1

[0018]FIG. 4 is a detailed view of a portion of a projection lens systemin accordance with an exemplary embodiment of the invention.

[0019]FIG. 4A provides a key between element surfaces and referencenumerals in FIG. 1.

[0020]FIG. 5 is a view of a projection lens system in accordance with asecond embodiment of the invention.

[0021]FIGS. 6A and 6B are detailed views of a portion of a projectionlens system in accordance with an exemplary embodiment of the projectionlens system in FIG. 5.

[0022]FIG. 7 is a view of a projection lens system with an illuminationsubsystem including an illumination relay lens system in accordance withan exemplary embodiment of the invention.

[0023]FIG. 8 is a view of a projection lens system with an illuminationsubsystem including an illumination relay lens system in accordance witha third embodiment of the invention.

[0024]FIG. 8A provides a key between element surfaces and referencenumerals in FIG. 8.

[0025]FIGS. 9 and 10 are views of portions of a projection lens systemwith an illumination subsystem in accordance with exemplary embodimentsof the invention.

[0026] FIGS. 11-14 are views of mounting apparatuses in accordance withexemplary embodiments of the invention.

[0027]FIG. 15 is a view of a projection lens system with an illuminationsubsystem in accordance with an alternative embodiment of the invention.

[0028]FIGS. 16 and 17 are views of details of portions of a projectionlens system in accordance with exemplary embodiments of the invention.

[0029]FIGS. 18 and 18A are views of a portion of a projection lenssystem in accordance with a fourth embodiment of the invention.

[0030]FIG. 19 is a side view of a display apparatus in accordance with afifth embodiment of the invention.

[0031]FIG. 20 is a side view of another display apparatus in accordancewith a sixth embodiment of the invention.

[0032] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Illustrative embodiments of the invention are described below. Inthe interest of clarity, not all features of an actual implementationare described in this specification. It will of course be appreciatedthat in the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

[0034] Improved projection lenses and an improved projection lenssystems are described in accordance with embodiments of the invention.The projection lenses and systems have utility in both front and rearprojection systems. The projection lens systems can include illuminationand relay lens subsystems. The projection lenses and systems may beemployed advantageously in so-called “folded” optical display systems.In the description and drawings of the projection lenses and systemsbelow, like reference numerals are indicative of like parts.

[0035] FIGS. 1-3 illustrate a reflection-based projection lens system 10in accordance with a first embodiment of the invention. The projectionlens system 10 includes a projection lens 12 having a first or frontlens unit 14 and a second or back lens unit 16. The front lens unit 14and the back lens unit 16 are separated by an air gap. The front lensunit 14 has overall zero, near-zero or weak (e.g., negative) opticalpower with an angular magnification to project over a wide field ofview. Other embodiments can have positive or negative optical powers forthe front lens unit 14. The second lens unit 16 has overall positiveoptical power. In the exemplary embodiment shown in FIGS. 1-3, thesecond lens unit 16 includes lens elements 18, 20, and 22 and the firstlens unit 14 includes lens elements 24, 26, 28, and 30. The lenselements 18, 20, 22, 24, and 26 are all positively powered lenses andthe lens elements 28 and 30 are both negatively powered lenses. The lenselements 18, 20, and 24 may be doublets and the lens elements 22, 26, 28and 30 may be meniscus lenses, although other lens types or powers couldbe used. Other arrangements and number of elements can be envisioned, aswill be appreciated by those skilled in the art having the benefit ofthe present disclosure. These other arrangements and number of elementsare included within the scope and spirit of the present invention.

[0036]FIG. 3 shows a larger view of the projection lens system 10 andthe projection lens 12. The projection lens 12 includes nine elements inthe exemplary embodiment. These nine elements include a reflectinglinear polarizer 32 in addition to the lens elements 18, 20, 23, 24, 26,28, and 30, and a clean-up element 34 (see FIGS. 2 and 3). In otherembodiments, the number of elements can be other than nine. The clean-upelement 34 can be an absorptive linear polarizer and is optional. Thereflecting linear polarizer 32 may be constructed of double brightnessenhancement film (DBEF), a variety of multilayer optical film (MOF),commercially available from Minnesota, Mining & Manufacturing Company.The reflecting linear polarizer 32 (e.g., MOF) is a substantiallynonabsorbing polarizer. Hence, it does not substantially absorb lightthat it transmits or reflects. An exemplary construction of thereflecting linear polarizer 32 is a sandwich of glass, optical cement,MOF, optical cement, and glass. The reflecting linear polarizer 32 isoriented to substantially reflect first linear polarization componentsof light of desired colors (which can be white light or substantiallywhite light) from a light source (not shown in FIGS. 1-3) toward therear lens unit 16 and to substantially transmit second linearpolarization components (orthogonal to the first) and reflect undesiredcolors. For example, the reflecting linear polarizer 32 can be orientedwith its high efficiency side toward the light source, such thatincoming S polarization light is reflected and P polarization light istransmitted. An additional reflecting linear polarizer (not shown)constructed of MOF, for example, may be placed in the path of thetransmitted light to reflect it back through the reflecting linearpolarizer 32 to the light source. Such operation is useful with certaintypes of high intensity discharge (HID) lamps (to be described in moredetail below) or other types of lamps for optical “pumping” of the lightsource to improve the efficiency of the light source for generating thedesired light components. These lamps are exemplified in prior co-ownedU.S. patent application Ser. Nos. 08/747,190, filed Nov. 12, 1996, byRichard M. Knox, entitled “High Efficiency Lamp Apparatus For ProducingA Beam Of Polarized Light,” and 08/771,326, filed Dec. 20, 1996, byWilliam B. Mercer, entitled “Polarized Light Producing Lamp ApparatusThat Uses Low Temperature Polarizing Film,” both incorporated byreference herein in their entirety.

[0037] A remote aperture stop 33 is located near the lens element 24between the reflecting linear polarizer 32 and the lens element 24, asshown in FIGS. 2 and 3. By positioning the aperture stop 33 remotelyfrom the polarizer 32 (i.e., by it being an accessible aperture stop),diffractive and/or other out-of-angle light can effectively be blockedfrom images. As a result, contrast can be improved by pupil apodizationfor contrast enhancement and/or other needs. The aperture stop 33 can bedesigned to be very close to (i.e., proximate to or just outside) thelens 24. In certain embodiments, a filter can be positioned in theaperture stop 33 to filter image light passing through, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure.

[0038] Whether the clean-up element 34 included in the exemplaryembodiment in FIGS. 1-3 is used may depend on desired image contrast.The clean-up element 34 can be sandwiched between two lens elements 24A,24B that make up the lens element 24, as shown in FIGS. 1-3, althoughother configurations are possible. The clean-up element 34 could becemented between the two elements 24A, 24B using a suitable opticalcement. In alternative embodiments, the clean-up element 34 could bepositioned at any appropriate location in the front group 14, forinstance: between the reflecting linear polarizer 32 and the lenselement 24; between the lens elements 24 and 26; between the lenselements 26 and 28; or between the lens elements 30 and a display screen36 (see FIGS. 1-3). In this last position, the clean-up element 34 maybe attached (e.g., by suitable optical cement) to the lens element 30 orit may be completely external to the lens 12. The clean-up element 34 ispreferably positioned in the front group 14 at locations where the imagelight is not substantially diverging or of large ray angles.

[0039] The first lens unit 14 may include at least one asphericalsurface or element (i.e., an asphere). For example, in the exemplaryembodiment shown in FIGS. 1-3, the lens elements 26 and 30 can beaspheres having aspheric surfaces 26A and 30A, respectively. In otherembodiments, different numbers of aspheric lens elements or surfaces canbe combined with non-aspheres, and exhibit analogous or similarperformance characteristics to the projection lens system 10. Moreover,additional embodiments exhibiting analogous or similar performancecharacteristics can include no aspheres and/or gradient index ordiffractive optical components, as will be appreciated by those skilledin the art having the benefit of the present disclosure. All of theseembodiments are included within the scope and spirit of the presentinvention.

[0040] In the exemplary embodiment shown in FIGS. 1-3, the projectionlens system 10 also includes imager 38 for color imaging and a chromaticseparator or beamsplitter 40. In general, as used herein, the imager 38is understood to mean one or more color imagers, for example, imagers38A, 38B, 38C for three-color imaging. Other numbers of imagers arepossible, for example, one, two, four, or more. The number of imagerswill depend, in general, on the specific implementation or design of theprojection lens system 10 and/or an illumination subsystem for theprojection lens 10. Examples of embodiments in which one or two imagerslike imagers 38A, 38B, 38C could be used are field sequential colorsystems, as will be appreciated by those skilled in the art having thebenefit of the present disclosure. For simplicity of presentation, insome of the drawings only one imager is shown, which is labeled as theimager 38 (see, e.g., FIGS. 2 and 3). In other drawings, all threeimagers 38A, 38B, 38C will be shown when the discussion warrants it orwhen easily drawn. In the view shown in FIG. 1, the imager 38C is notvisible as it is obscured by the chromatic separator 40. A separatecover glass 39A, 39B, 39C (indicated generally as numeral 39 in thedrawings showing only the imager 36) is included for each of therespective imagers 38A, 38B, 38C. There is a small (not shown) air gapbetween each cover glass 39A, 39B, 39C and the respective imagers 38A,38B, 38C. In other embodiments, the cover glass 39A, 39B, 39C may beintegrated with the imager 38A, 38B, 38C, there may be no air gap, orthere may be no cover glass at all. Each of the color imagers 38A, 38B,38C may be LCD imagers, such as ferroelectric LCD (FLCD) imagers, orother forms of imagers.

[0041] Any appropriate chromatic separator can be employed as thechromatic separator 40. In FIGS. 1-3, the chromatic separator is shownsimply as a block. FIG. 4 offers a view of the imagers 38A, 38B, 38C andthe chromatic separator 40 in an exemplary embodiment. The front lensunit 14, the rear lens unit 16, and the reflecting linear polarizer 32are not in detail in FIG. 4. The chromatic separator in FIG. 4 is aPhilips prism, which is discussed further below. The chromatic separator40 splits the incoming white light received from an illuminationsubsystem (not shown in FIGS. 1-3) into three color bands, for example,the red, green, and blue primary colors, as generally indicated byrespective numerals 42A, 42B, and 42C in FIGS. 2-4. The illuminationsubsystem includes the light source and the incoming white light isreceived by the chromatic separator 40 via reflection from thereflecting linear polarizer 32, as discussed above. The incoming whitelight may be substantially white or quasi-white light. Quasi-white lightis defined to be light from a light source that is deficient in itsoutput in one or more colors (or wavelength bands) of the visiblespectrum. Substantially white or quasi-white light will be referred toherein simply as white light. The chromatic separator 40 separates theprimary colors in the incoming white light in the exemplary embodimentsshown in FIGS. 1-4. The color-separated light components 42A, 42B, 42Care directed along different paths to corresponding ones of the imagers38A, 38B, 38C.

[0042] One way to direct the color-separated light 42A, 42B, 42C is touse the well-known Philips prism as the chromatic separator 40, asalready mentioned. The Philips prism is a type of chromatic separatorthat includes one or more prism elements, for example, prism elements44A, 44C, and an optional cover 44B, as shown in FIG. 4. Each of theprism elements 44A, 44C includes a highly reflective, multilayeredcoating (e.g., coatings 44D, 44E) designed to substantially reflect ortransmit particular colors of light to separate the colors. Each of thecoatings 44D, 44E preferentially reflect or transmit a color that isdistinct from the colors reflected or transmitted by the multilayeredcoating on the other prism element. In other words, the coating 44D is,in general, different, and reflects and transmits differently, than thecoating 44E. In other embodiments, the chromatic separator 40 could takeother forms that function analogously or similarly to the Philips prism,such as the well known X-cube beamsplitter.

[0043] In typical use, each of the three color imagers 38A, 38B, 38Creceives the color-separated light or bands of light 42A, 42B, 42Cderived from the illumination subsystem (i.e., from illumination light)and reflects back a corresponding color-separated image imparted on eachcolor band, as indicated schematically by numerals 46A, 46B, and 46C inFIGS. 2-4. The imagers 38A, 38B, 38C, if they are FLCDs, twisted nematicLCDs, or other types of spatial light modulators, each impart therespective color-separated image under control derived from an externalvideo or other control signal (not shown). The control signal can beimplemented as a temporal electrical modulation of electrooptic statesof individual pixels (not shown) that are defined in the imagers 38A,38B, 38C. Each pixel is individually electrically addressable forcontrol of its states. One state (e.g., an “on” state) rotates (i.e.,retards) the polarization of incoming light by substantially 90 degrees.Retardation occurs because the light impinging on the pixel makes adouble pass through a quarter-wave optical thickness of the pixel withan intervening reflection. A reflector located behind the pixel orforming a back part of the pixel provides the reflection. The otherstate (i.e., an “off” state) does not substantially rotate thepolarization before or after reflection during the double pass.Projectable gray levels are achievable at intermediate states betweenthe on and off states, for example, if the imagers 38A, 38B, 38C are thetwisted nematic LCDs, which have a variable birefringence with appliedvoltage. Intermediate voltage values between the on and off statevoltage values can produce analog gray scale. The FLCDs are bi-stabledevices and hence they would only have the two states discussed (i.e.,on and off).

[0044] At any instance in time during image formation, a particularelectrical on and off state pixel pattern corresponds to the imageinformation that is imparted on the light 46A, 46B, 46C upon reflectionfrom the imagers 38A, 38B, 38C. This pattern is transformed into apattern of polarization states of different bundles of the light 46A,46B, 46C (i.e., into polarization-encoded bundles of the reflected light46A, 46B, 46C). The color-separated image information in the image light46A, 46B, 46C is then combined by the color separator 40. The bundles ofthe light 46A, 46B, 46C traveling from the rear unit 16 toward the frontunit 14 are then selected according to their polarization state by thereflecting linear polarizer 32. Image light that had its polarizationrotated substantially by 90° by the imagers 38A, 38B, 38C issubstantially transmitted through the reflecting linear polarizer 32 aslight 48. Light (not shown) whose polarization was not substantiallyrotated is reflected by the reflecting linear polarizer 32 and out ofthe projection lens 12, back toward the illumination subsystem. Thereflected light travels essentially the same path in reverse of the paththat the incoming light took from the light source in the illuminationsubsystem. This reflected light could be used for optical pumping of thelight source for improved efficiency in the illumination subsystem, insimilarity to the discussion above.

[0045] The transmitted light 48 has substantially the secondpolarization orthogonal to the previously desired (first) polarizationof incoming light that was reflected by the reflecting linear polarizer32 toward the imagers 38A, 38B, 38C. The light 48, therefore, passesthrough the reflecting linear polarizer 32 and through the clean-upelement 34, if present. Characteristic directions of the clean-upelement 34 and the reflecting linear polarizer 32 are aligned for thistransmission, and the clean-up element 34 selects the polarizationfurther. The light 41 then passes through the front lens unit 14 towardthe screen 36 as image light 49, which forms a full color imageprojected thereon (see FIGS. 1 and 2). The nominal throw of theprojection lens 12 to the screen 36 (i.e., the distance between them) isapproximately 447 mm in air in the exemplary embodiments in FIGS. 1-4.Other embodiments can be designed with different throw distances. Themagnification to the screen 36 is approximately 26, although othermagnifications could be designed, as will be appreciated by thoseskilled in the art having the benefit of the present disclosure. Themagnification to the screen 36 is approximately 26, although othermagnifications could be designed, as will be appreciated by thoseskilled in the art. With the use of optical designs of different angularmagnifications in the front and rear groups, the light can be imagedonto screens of different sizes. For example, the front lens unit canexhibit high angular magnification for wide field projection.

[0046] The projection lens 12 advantageously exploits the light outputfrom the illumination subsystem that is imaged onto the imagers 38A,38B, 38C by being telecentric or substantially telecentric in objectspace. Moreover, the numerical aperture (NA) of the projection lens 12is nominally high. In the exemplary embodiments shown in FIGS. 1-4, theprojection lens 12 has an NA of approximately 0.1786, which isequivalent to an F/# of approximately 2.8 in air. The projection lens 12could be designed to have other NA values.

[0047] Table 1 summarizes nominal projection lens 12 data for theexemplary embodiments shown in FIGS. 1-4. TABLE 1 GENERAL PROJECTIONLENS DATA Operating Temperature (° C.) 0°-60° C. Stop S18 [FIG. 4A andTable 2] Stop Diameter 16 mm Eff. Focal Length 18 mm Object Space NA0.17860 Image Diagonal 542 mm Magnification 26.25 Entrance PupilPosition ∞ (telecentric) Exit Pupil Diameter 6.5 mm Exit Pupil Position−473 from image Object Diagonal 20.6 mm Wavelength Band Visible LensUnits Millimeters

[0048] Table 2 is a summary of the projection lens 12 surface data forthe exemplary embodiments shown in FIGS. 1-4. The columns in Table 2 arefor surface number, surface radius, thickness (i.e., distance betweenthe surface indicated in a row of Table 2 and the surface indicated inthe next row), glass/material (e.g., glass or other material or materialparameters), diameter, and conic (for aspheric surfaces). FIG. 4Aassociates the element surfaces in the second column of Table 2 with thenumerical elements in the first column of Table 2 and shown in FIGS.1-3. Surfaces S13-S16 represent interior surfaces of the exemplaryglass/optical cement/MOF/optical cement/glass embodiment of thereflecting linear polarizer 32 and are not specifically identified inFIG. 4A. Table 2 includes surfaces S19-S22 for the clean-up element 34,although the clean-up element 34 is optional.

[0049] Table 3 includes higher order aspheric coefficient entries forthe aspheric surfaces S23 and S27 of the lens elements 26 and 30,respectively, decentering information for the surface S19, and tiltinformation for the surfaces S12 and S17.

[0050]FIG. 5 shows a projection lens system 50 in accordance with asecond embodiment of the invention. The system 50 is similar to theprojection lens system 10, and is a variation of the projection lenssystem 10. A projection lens 52 includes a front or first lens unit 14′,which is similar to the lens unit 14 in the lens 12. The front lens unit14′ includes lens elements 24′, 26′, 28′, and 30′. An optional clean-upelement 34′ can be sandwiched between lens elements 24A′ and 24B′ ofwhich the lens element 24′ is constructed. The front lens unit 14′ alsoincludes a remote aperture stop 33′. The elements 24′ (24A′ and 24B′)26′, 28′, 30′, 33′, and 34′ are analogous or similar to the elements 24(24A and 24B), 26, 28, 30, 33, and 34, respectively, in the projectionlens 12. The system 50 further includes the imager 38 and the chromaticseparator 40. TABLE 2 PROJECTIONS LENS SURFACE DATA SUMMARY Dwg. ElementNo. Surf No. Radius Thickness Glass/Material Diameter Conic 38 OBJECT ATInfinity 1.1 ZKN7 22.4 IMAGER S1 Infinity 0.8 22.4 40 S2 Infinity 40 BK736.72 S3 Infinity 1.5 36.72 18 S4 Infinity 2.5 SF11 37.6 S5 58.57127 8.8SK5 37.6 S6 −44.27827 0.5 37.6 20 S7 102.6493 9.4 SK5 37.6 S8 −34.326822.5 SF11 37.6 S9 −101.7558 0.5 37.6 22 S10 45.26133 5.1 BK7 36 S11142.304 17.704 36 S12 — 0 — S13 Infinity 0.7 BK7 37.5 32 S14 Infinity0.125 index 1.580000 37.1 Abbe number 58.000 S15 Infinity 0.7 BK7 37 S16Infinity 0 36.6 S17 — 12.596 — 33 Stop S18 — 0 16 S1 27.40444 4 BASF2 20S2 Infinity 0.125 index 1.450000 20 Abbe number 58.000 24, S3 Infinity2.65 BASF2 20 34 (S20, S21) S4 51.24619 19.2 20 26 S5 30.24837 4.5ACRYLIC 29.4 0.41940 S6 33.3 10.5 29.4 28 S7 −14.45517 6.8 BK7 24.8 S8−35.82717 1.55 37.6 30 S9 −26.2 4.3 ACRYLIC 45 −1.5309 S10 −66 447 45 36SCREEN Infinity — 577.872 IMAGE S29

[0051] TABLE 3 Dwg. Element Aspheric No. Surface A(y⁴) B(y⁶) C(y⁸)D(y¹⁰) 26 S23 −1.28E-5 −2.83E-8 4.1E-11 −5.0E-14 30 S27 4.67E-6 1.77E-8−4.57E-12 −2.92E-14

[0052] The front lens unit 14′ is laterally adjustable as a group withrespect to the remainder of the projection lens 50. Lateral adjustmentcan be made by decentering along X and Y axes in a right-handedcoordinate system 54 shown in FIG. 5. The direction of motion is alsogenerally indicated by the double-headed arrow 56 parallel to the Y axisand the orthogonal arrowhead/tail 58 parallel to the X axis.

[0053] The purpose of decentration is to mitigate possible effects ofmanufacturing tolerances within the projection lens 50 to improve imagequality. The mechanism for decentration in the embodiment shown in FIG.5 could be implemented in various configurations, as will be appreciatedby those skilled in the art having the benefit of the presentdisclosure. One exemplary mechanism is shown in FIGS. 6A and 6B inaccordance with an embodiment of the invention. FIGS. 6A and 6B show aportion of a housing 60 of the front lens unit 14′ of the projectionlens 52. In this embodiment, the front lens unit 14′ is constructed as amodular barrel 62 that installs into the housing 60 in a directiongenerally indicated by arrow 64. When the front lens unit 14′ is fullyinserted into the housing 60 (FIG. 5B), flat 66 rests on flat 68. Thehousing 60 can be articulated along axes 70 and 72 by suitableadjustment known in the art (e.g., by screw adjustment). In oneexemplary embodiment, the barrel 62 is manipulated with an externaldevice (not shown), such as a screwdriver, until the opticalcharacteristics of the projection lens 52 are measured for bestperformance. The barrel 62 is then glued in place with an appropriateglue.

[0054] In accordance with an embodiment of the invention, anillumination subsystem includes an illumination relay lens system forintroducing light from a light source to the projection lenses 12, 52.One exemplary embodiment including such an illumination subsystem 74 isillustrated in FIG. 7. An illumination relay lens system 76 receiveslight from a light source 78A. The illumination relay lens system 76directs light output from the light source 78A to the reflecting linearpolarizer 32 in the projection lenses 12, 52. The rear lens unit 16 inthe projection lenses 12, 52 is common to light paths of theillumination subsystem 74 (or other types of illumination subsystemsdiscussed herein) and the projection lens systems 10, 50. Themagnification of the illumination relay lens system 76 is approximatelytwo in one embodiment. In other embodiments, the illumination relay lenssystem 76 may include one or more aspheres (e.g., constructed of apolymer, such as acrylic), and may have different magnifications andelement powers.

[0055] In FIG. 7, the light source 78A includes a lamp 80A and a lamppower drive or power source (not shown). The lamp 80A may be driven byelectric arc, radiofrequency (rf) energy, microwave, or like powersource and include equipment or hardware (not shown) for coupling powerto the light emitting material of the lamp 80A. The lamp 76A can be oneof the lamps described in the aforementioned U.S. patent applicationSer. Nos. 08/747,190 or 08/771,326, or in U.S. Pat. Nos. 5,404,026,entitled “Lamp Including Sulfur,” and 5,606,220, entitled “Visible LampIncluding Selenium or Sulfur,” both issued to Dolan et al., which areincorporated by reference herein in their entirety.

[0056]FIG. 8 shows a projection system and an illumination subsystem inaccordance with a third embodiment of the invention. A light source 78Bis similar to the light source 78A and includes a lamp 80B (similar tothe lamp 80A) and a lightpipe (e.g., a tapered lightpipe or TLP) 82,which is a type of lightguide. The lamps 80A, 80B and the TLP 82 will bediscussed further below. Like Table 1, Table 4 summarizes generalprojection lens 12, 52 data and illumination relay lens system 74 datafor the embodiment shown in FIG. 8. Table 5 is a summary of theprojection lens 12 (and 52) surface data for FIG. 8, in similarity toTable 2.

[0057]FIG. 8A associates the element surfaces in the second column ofTable 5 with the numerical elements in the first column of Table 5 andshown in FIG. 8. Surfaces S13-S18 represent interior surfaces of theexemplary embodiment of the reflecting linear polarizer 32 and are notspecifically identified in FIG. 8A, as similarly discussed above forFIG. 4A. No meaning should be attached to the use of similar elementsurface numerical labels between the embodiments shown in Table 2 (andFIG. 4A) and Table 5 (and FIG. 8A). TABLE 4 GENERAL LENS DATA No.Surfaces 30 Temperature (° C.) 0°-68° C. Object Space N.A. 0.32 Eff.Focal Length −172 mm Working F/# 2.65 Stop Diameter 16 mm ParaxialMagnification −1.78 Object Height in Millimeters 12.5 mm diagonalPrimary Wavelength 0.556 microns Lens Units Millimeters

[0058] The illumination relay lens system 76 is designed to accommodatethe extent or size of the light output from the TLP 82. In a particularembodiment, the TLP 82 and the imager 36 are not substantiallyadjustable relative to each other while their adjustment can be made inother embodiments or in other ways in still other embodiments. Forexample, an illumination field stop 83 (see FIGS. 7, 7A, and 8) can belaterally adjusted to allow light passing from the light sources 78A,78B to the imager 38 to be centered on the imager 38. The field stop 83can be a rectangular field stop.

[0059] In the exemplary embodiment shown in FIG. 8, a pre-polarizer 86is also included in an aperture stop 84. The pre-polarizer 86 can be amulti-layered or sandwiched structure in a heat-sink frame, such aslayers of DBEF (or MOF), glass, air, sapphire, and an absorptionpolarizer (e.g., with optical cement in between each adjacent layer).The sapphire acts as a heat collector and the pre-polarizer 86 can beAR-coated. With this construction, the sapphire layer may be usedadvantageously as a heat sink, depending on the design of the lightsource 78A, 78B. The MOF layer of the pre-polarizer 86 may be used toreflect light of an undesired polarization (i.e., polarization notaligned for reflection to the imager 38 by the reflecting linearpolarizer 32) back to the lamp 80B for optical pumping, as discussedabove, as well as to limit the amount of light absorbed by theabsorption polarizer to minimize heating effects. On the other hand, theMOF layer transmits light of the desired polarization (i.e.,polarization aligned for reflection to the imager 38 by the reflectinglinear polarizer 32).

[0060] The illumination relay lens system 76 may also include an IR/UVfilter or coating 88 on a lens 90. Infrared filtering can reduce orsubstantially mitigate detrimental thermal effects from high poweredlamps in imaging systems. Ultraviolet filtering can reduce orsubstantially mitigate degradation of optical bonding materials (e.g.,optical cements or epoxies) if they are used in the projection lenssystems 10, 50. The IR/UV filter 88 shown in FIG. 8 reflectsnear-visible IR radiation from the light source 78A, 78B away from theprojection lenses 12, 52 and back toward the lamp 80B. The IR radiationabove approximately 1.2 microns is absorbed by the absorption polarizerin the IR/UV filter 88. In alternative embodiments, only an IR or a UVfilter (i.e., not both), or no filter, may be used. The IR/UV filter 88(or separate IR or UV filters) could be in other positions, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure. For example, the filter 88 could be a UV filter or aUV coating on the lens 90 and an IR mirror could be placed near or inthe middle of the illumination relay lens system 76 between the lenses74A and 74B in FIGS. 7, 8 and 8A. With an IR (hot) mirror, UV could beabsorbed and so there may be no need for a separate UV filter orcoating. TABLE 5 ILLUMINATION RELAY SYSTEM SURFACE DATA SUMMARY Dwg.Element No. Surface No. Radius Thickness Glass/Material Diameter Conice.g., 82 OBJECT (e.g., Infinity 0.005 12.5 0 TLP OUTPUT) 90 S1 Infinity2.3 SILICA 16 0 90, 90A S2 −25.2 2.877403 16 0 S3 Infinity 4.5 BK7 20 0S4 −18.59391 14.45 20 0 76 S5 Infinity 5.5 BK7 22 0 S6 −27.77859 0.5 220 S7 Infinity 3.7 BK7 22 0 S8 −34.7788 2.22 22 0 86 S9 Infinity 1.5 BK718 0 S10 Infinity 0 18 0 84 STOP 11 Infinity 14.35 — 16 — S12 — 0 — — —S13 Infinity 0.7 BK7 36.52763 0 S14 Infinity 0 MIRROR 37.41535 0 S15Infinity −0.7 BK7 37.41535 0 32 S16 Infinity 0 38.30535 0 S17 — 0 — — —S18 Infinity −18.225 23.73374 0 S19 — 0 — — — 22 S20 142.304 −5.1 BK7 370 S21 45.26 −0.5 37 0 S22 −101.755 −2.5 SF11 37.5 0 20 S23 −34.326 −9.4SK5 37.5 0 S24 102.649 −0.5 37.5 0 S25 −44.278 −8.8 SK5 37.5 0 18 S2658.571 −2.5 SF11 37.5 0 S27 Infinity −1.5 37.5 0 40 S28 Infinity −40 BK736.72 0 S29 Infinity −0.8 36.72 0 39 S30 Infinity −1.1 ZKN7 24.75913 038 IMAGE AT Infinity — 22.93756 0 IMAGER S31

[0061] The TLP 82 is four-sided, pyramidal-shaped, and rectangular incross-section, with flat sides and ends in the exemplary embodimentshown in FIG. 8. The TLP 82, having this structure, is used to“condition” the light, although other shapes could be used. The TLP 82accepts light from the lamp 80B and guides the light substantially bytotal internal reflection (TIR), as will be appreciated by those skilledin the art having the benefit of the present disclosure. The lightreceived from the lamp 80B is multiply reflected within the TLP 82 as itundergoes TIR and is output by the TLP 82 to the illumination relay lenssystem 76. The TLP 82 exhibits TIR because of its shape and its opticaland material properties, and because of its orientation for receivinglight from the lamp 80B. In FIG. 8, the TLP 82 is shown, bonded to,layered with, or otherwise attached to a lens element 90 (e.g., apositive lens). The lens 90 can also be integral with the TLP 82 inother embodiments. In the exemplary embodiment shown in FIG. 8, the lenselement 90 includes a lens surface 90A bonded to the TLP 82 with theUV/IR filter coating 88 in between. It will be appreciated by thoseskilled in the art having the benefit of the present disclosure that theIR/UV filter or coating 88 could be disposed at other positions withinthe system 76 or on the surface of the lens element 90 away from the TLP82. Embodiments, for example, as shown in FIGS. 9 and 10, in which thelens surface 90A is integral with the TLP 82 are simple, low cost, andradiometrically efficient.

[0062] In operation, the relay system 76 images the light output fromthe TLP 82 onto the imager 38. Light is both homogenized and controlledin solid angle by the TLP 82 to allow for simple imaging onto the imager38 with little loss. The TLP 82 conditions the light output from thelamp 80B to become substantially telecentric light at the imager 38. Thelight from the TLP 82 is provided at the right NA to the illuminationrelay lens system 76 to produce the near-telecentric light at the imager38. In alternative embodiments, a condenser, which is also a lighthomogenizer, could be used instead of the TLP 82 and the relay system76. The condenser would form an image of the light source at theentrance pupil of the projection lens, thereby matching the illuminationsystem to the projection system.

[0063]FIG. 11 illustrates an exemplary mounting apparatus for holdingthe TLP 82 in accordance with an embodiment of the invention. In FIG.11, the lens 90 is bonded to the TLP 82 with the IR/UV coating 88 inbetween. Bonding is made using a suitable optical adhesive. The lens 90is mounted in the illumination subsystem 74 by mount 92, shown incross-section in FIG. 11, which can completely encircle the lens 90along an edge 94 of the lens 90. The lens 90 could be glued ormechanically retained within the mount 92. The mount 92 is completelyoutside the light cone 96 passing through and out of the TLP 82. Theapparatus shown in FIG. 11 is a desirable embodiment, because light lossdue to loss of TIR can be reduced or avoided if the TLP 82 were notmounted and, therefore, not contacted on its side 98, or on its end 100.A physical mount 102 at the end 100 is optional. The mount 102 cancompletely or substantially decouple the TLP 82 from the lamp 80B, whichmay afford prevention or reduction of possible physical and thermaldegradation.

[0064]FIGS. 12, 13, and 14 show some variations in ways to mount the TLP82 in accordance with alternative embodiments of the invention. FIG. 12illustrates a mount 106 similar to the mount 92 (e.g., it is completelyoutside the light cone 90 passing through and out of the TLP 82).Physical contact is made between the lamp 80B and/or its housing 108 andthe TLP 82. The TLP 82, the mount 106, and the lens 90 are positioned sothere is, in general, a force directed toward the lens 90 from the lamp80B. No adhesive is required between the lens 90 and the TLP 82 orbetween the lens 90 and the mount 106. The force pushes the TLP 82against the lens 90 in the mount 106. FIG. 13 shows a detail of thecontact made between the TLP 82 and the lens 90. FIG. 14 shows anotherdetail with the TLP 82 including a ground corner region 109 thatsubstantially conforms to the curvature of the lens 90. The exemplaryembodiment shown in FIG. 14 may provide improved fragility in the cornerregion 109, both for the TLP 82 and the lens 90. The UV/IR coating 88(or only one of them, as discussed above), although not shown in FIGS.12 or 13, can be disposed in between the TLP 82 and the lens 90.

[0065] In an illumination subsystem 110 shown in FIGS. 15A and 15B, inaccordance with another alternative embodiment of the invention, a lightfunnel 112A/compound parabolic concentrator (CPC) 112B in a combination112, having reflective inner surfaces 114 and 115, or other type orshaped lightguide may be employed instead of the TLP 82 in a relaysystem. A lightpipe (e.g., a non-tapered light homogenizer) 113 isincluded in the subsystem 110 to homogenize the light received from alight source (e.g., 80A or 80B) that passes through the combination 112on its way to an illumination relay lens system (e.g., the system 76).The light funnel portion of the combination 112 is a funnel-shaped,reflecting optical element. Devices similar to the CPC 112B and the TLP82 are described in U.S. Pat. Nos. 5,237,641, 5,243,459, 5,303,322,5,528,720, 5,557,478, 5,610,768, and 5,594,830, which are incorporatedby reference herein in their entirety. The region 117 between the funnel112A and the CPC 112B is a region of high light energy. The cone angle ((see FIGS. 15A and 15B) of the funnel 112A, which determines the coneangle of the light through the lightpipe 113, is preserved at the outputof the lightpipe 113, as shown in FIG. 15B. The light output from theCPC 112B is telecentric or substantially telecentric. The angle (determines the angle of the cone of light at the output of theillumination relay lens system, and is related to its telecentricity. Incertain alternative embodiments, the funnel 112A is not included withthe CPC 112B. In still other alternative embodiments, the TLP 82 can bereplaced with a system of lenses that may include one or more asphericsurfaces and/or gradient index or diffractive optics that freely imagelight from the lamp 80B to the imager 38. Such alternative embodimentsalso provide well behaved, substantially telecentric cones of light tothe imager 38. The TLP 82, as well as the illumination relay lens system76, could also both be replaced by a completely different illuminationrelay lens system of another design, as will be appreciated by thoseskilled in the art having the benefit of the present disclosure. All ofthese embodiments image light from the lamp 80B onto the imager 38,providing substantially telecentric and uniform light. Moreover, the TLP82, as well as these other types of lightguides and relay lens systems,beneficially allows for the use of an arc lamp, such as a metal halidelamp, or other lamp types. They also provide high efficiency for highillumination brightness and uniformity.

[0066] As mentioned above, in accordance with embodiments of theinvention, the illumination subsystem 74 and the imager 38 light can beadjusted relative to each other. In one embodiment, the position of theTLP 82 can be adjusted by simple mechanical adjustment (e.g., by a screwadjustment) relative to the illumination relay lens system 76. Forexample, the TLP 82 can be laterally or angularly adjusted relative tothe system 76, which controls the cones of the light impinging on theimager 38 to also move laterally or angularly, as will be appreciated bythose skilled in the art having the benefit of the present disclosure.

[0067] In another embodiment shown in FIGS. 16 and 17, the reflectinglinear polarizer 32 can be adjusted about one or more axes of rotationto adjust the illumination subsystem 74 (and hence, the TLP 82) and theimager 38 relative to each other. In this embodiment, the adjustablefield stop 83 is not needed to adjust the illumination on the imager 38,and is not necessarily present. This will allow the substantiallytelecentric light received from the illumination subsystem 74 via theillumination relay lens system 76 to be adjusted on the pixel faces ofthe imager 38. Adjustment of the polarizer 32 can be used to optimizethe coupling of light between the output of the TLP 82 and the imager38. Moreover, as with adjustment of the front lens unit 14′ (see FIG.5), adjustment of the polarizer 32 can be used to compensate formanufacturing or mounting tolerances.

[0068] In more detail, FIGS. 16 and 17 show a beamsplitter adjustmentdevice 130 that includes adjustment screws 132A, 132B, adjustment cams134A, 134B (e.g., 2:1 cams), an adjuster 136, and a pivot 138 (not shownin FIG. 17). The adjustment device 130 can be constructed of moldedplastic components attached to the inside of the projection lenses 12,52 (not shown in detail in FIG. 16). The adjustment screws 132A, 132Bcontact the cams 134A, 134B, respectively. The cams 134A, 134B rotateagainst the adjuster 136 to rotate or tilt the polarizer 132. Turningboth of the adjustment screws 132A, 132B causes a top 32A of thepolarizer 32 to tilt up to approximately 1° from a vertical plane asgenerally indicated by arrow 140. Turning only the adjustment screw 132Acauses the polarizer 32 to rotate up to approximately 1° from a 45°plane, as generally indicated by arrow 142. The adjuster 136 alsoreturns the cams 134A, 134B in the corner (i.e., the adjuster 136 holdsthe cams 134A, 134B in place inside the projection lenses 12, 52).Springs 144A, 144B bias the adjustment screws 132A, 132B against thecams 134A, 134B. Light received by the polarizer 32 can be steered tothe imager 38 by adjustment of the adjustment screws 132A, 132B tooptimally illuminate the imager 38. The rear lens unit 16 is located inthe direction indicated by arrow 146 in FIGS. 16 and 17.

[0069] Other modifications besides mechanical adjustment mechanisms foroptimizing illumination of the imager 38 can be made to the projectionlenses 12, 52 based on other considerations. For example, the lenses 12,52 and other nominal lens designs are multi-color projection lenses,which may exhibit residual color fringing, also referred to as lateralcolor. Color fringing is a result of a higher or a lower magnificationfor one or more colors compared to the other colors in the opticalsystem. For example, red and blue light may image at higher or lowermagnification than green light. For light exhibiting lower red and bluemagnification, adding a very weak, negatively powered element near thegreen imager (e.g., 38A, 38B, or 38C) in the green light path or channelcan decrease the magnification of the green light to compensate for themagnification differences. Thus, the red, green, and blue light can besubstantially and simultaneously matched in magnification to the othercolors. Other similar or analogous embodiments for correcting colorfringing also use one or more weak lens elements for the red and/or theblue channel in addition to the green channel, or instead of the greenchannel. More than one color and/or other colors besides red, green, orblue may be corrected. These embodiments and other embodiments that useweak, positive lenses or combinations of weak negative and positive lenselements to decrease or increase the magnification in one or morechannels to correct color fringing are included within the scope andspirit of the present invention.

[0070] A system for correcting color fringing in one color channel(e.g., the green channel) is shown in FIG. 18 and in FIG. 18A in moredetail in accordance with a fourth embodiment of the invention. Aprojection lens system 150, which is a variation of the projection lenssystems 10 and 50, includes a projection lens 152 (shown schematicallyas a cut-away block in FIG. 18). The system 150 may be similar to thesystems 10, 50 (e.g., including the adjustment mechanisms describedabove), except for the addition of a weak correcting element 154 (e.g.,a lens). The weak correcting element 154 is disposed between the colorseparator 40 (also see FIG. 1-3) and the one imager of the imagers 38A,38B, or 38C (in this case imager 38B is illustrated) that is being usedto impart the particular color image on the incoming light that needs tobe corrected for lateral color. The element 154 can be bonded to (e.g.,with an appropriate optical cement) either the chromatic separator 40 orto the cover glass associated with that particular imager.Alternatively, the element 154 can be disposed and held in place betweenthe chromatic separator 40 and the imager by an appropriate mount. Thecorrecting element 154 includes at least one curved surface 154A, whichexhibits optical power.

[0071] The weak correcting element 154 is used to bring the color to becorrected (e.g., green) into substantial coincidence with the othercolors (e.g., red and blue light) upon recombination of the light in thecolor separator 40 on its way to the front lens units 14, 14′. This isillustrated in FIG. 18 by considering the following. Pixel A on thegreen imager 38B corresponds to pixel C on the blue imager 38C and topixel D on the red imager 38A. The projection lens 152 has the lateralchromatic aberration, which is the change in magnification for differentcolors. The green channel has a larger magnification than the red andthe blue channels. To correct this difference in magnification in theprojection lens 152, the weak negative lens 154 has been added in frontof the green imager 38B. The light 46B coming from the pixel A, which isthe correct light, is redirected by the lens 154 such that it appears asif it is coming from the pixel B, which is closer to the center of theimager 38B. In other embodiments, gradient index, aspheric lenses, ordiffractive optics could be used for the weak correcting element 154.For example, diffractive optics in the channel for the color to becorrected in the projection lens could be used (e.g., in the rear unit)instead to correct color fringing. Another way to accomplish this is touse a diffractive optical element common for all three color channels.

[0072] In the exemplary embodiment shown in FIG. 18, the weak correctingelement 154 is used to reduce or eliminate color fringing in the greenchannel, which uses the imager 38B having the cover glass 39B (althoughinclusion thereof is dependent on the particular of the design imager38B). The other colors have corresponding color images imparted thereonin the projection lens 152 by operation of the imagers 38A, 38C. Inother embodiments, weak correcting elements may be employed to correctthe colors associated with the other imagers (i.e., 38A and 38C).

[0073] Another consideration for the projection systems 10, 50, 150 isthe use of high power light sources, such as those described in theaforementioned U.S. Pat. Nos. 5,404,026 and 5,606,220 and in U.S. patentapplication Ser. Nos. 08/747,190 and 08/771,326, when aspheric elementsor aspheres are also included in the lenses 12, 52, 152. Aspheres arefrequently constructed of a polymer or polymer materials. Certainpolymer materials, although exhibiting excellent optical properties, canalso exhibit detrimental thermal effects due to temperature changes thatoccur in the materials when high power light passes through the polymerasphere. High power light can negatively impact the projected imagesthrough these temperature changes. For example, aspheres constructed ofacrylic material are subject to changes in refractive index withincreasing temperature. This is because of a high coefficient of thermalrefractive index change. As a result, focus can change with temperature.Clever design using aspheres, however, may enable this thermal effect tobe substantially canceled or eliminated, which is termedathermalization. The projection systems 10, 50, 150 shown in FIGS. 1-3,5 and 18 implement athermalization. The projection lenses 12, 52, 152provide athermalization by carefully designing them to use aspheres(e.g., the lens elements 26 and 30) and to shift (e.g., positive)optical power from the aspheres (which can, therefore, have weak opticalpower) to the other elements that pass the high power light earlier asthe light proceeds through the front lens units 14, 14′. These otherelements are the lens elements 18, 20, 22, or 24, which allow theaspheric element 26 to be designed with lower (e.g., positive) opticalpower than might otherwise be required. These other elements can beconstructed of glass, which is less subject to thermal refractive indexchanges than are the polymer aspheres. Related detrimental thermalimaging effects are thereby avoided or prevented. The same thermalproblem is unlikely to occur with negatively powered lens elements, suchas the aspheric element 30, where the beam diameter is small for anyfield position. If the aspheres were made of glass instead of polymer,such thermal effects could likewise be reduced or eliminated. In thislatter case, the glass aspheres would not necessarily be limited tohaving weak optical power.

[0074] The remote aperture stop projection lenses 12, 52, and 152described herein offer improved optical performance. The benefits ofhaving a remote aperture stop include better exclusion of out-of-anglelight than conventional lens designs. The remote aperture stopprojection lenses 12, 52, 152 also provide wide fields of view, aretelecentric, and exhibit excellent resolution and near zero distortion.The projection lenses 12, 52, 152 are compact and manufacturable. Theyalso minimize ghost image formation and offer improved uniformity ofscreen brightness. For athermalization purposes, strategic use can bemade of two (or more) aspheric surfaces (e.g., constructed of acrylicmaterial) in these compound lenses that are otherwise composedsubstantially of spherical glass surfaces and materials. Moreover, theprojection lens 152 can additionally provide substantial lateral colorcorrection for color imaging with light passing through the same opticalcomponents (aspheric glass elements are also feasible).

[0075] The projection lens systems 10, 50, 150 may be similar to imageengines described in prior, co-owned U.S. patent application Ser. No.08/730,818, filed Oct. 17, 1996, by Richard M. Knox, entitled “ImageProjection System Engine Assembly,” which is incorporated by referenceherein in its entirety. The projection lens systems 10, 50, 150 may beadvantageously employed in front or rear projection systems, such as“folded” or “folded optics” display apparatuses. The display apparatuses200 and 250 shown in FIGS. 19 and 20, respectively, are examples ofthese folded apparatuses in accordance with fifth and sixth embodimentsof the invention. One or more imager configurations (e.g., that use twoor three imagers, like the imagers 38A, 38B, 38C) using color liquidcrystal filters may be employed. The display apparatuses 200 and 250 canbe part of a computer monitor or television display. They are similar tothe projection systems described in prior, co-owned U.S. patentapplication Ser. No. 08/581,108, filed Dec. 29, 1995, by Richard M.Knox, and in European Pat. app. No. 96309443.8, EP0783133A1, filed Dec.23, 1996, by Richard M. Knox et al., published Jul. 9, 1997, bothentitled “Projecting Images,” which are incorporated by reference hereinin their entirety. Such a “double bounce” geometry offers distinctadvantages. For instance, the folded optical paths in the displayapparatuses 200, 250 enable the size of the apparatuses 200, 250 to bereduced compared to other types of display apparatuses. This isillustrated in FIGS. 19 and 20, where the “footprint” dimensions “L” and“L′,” respectively, may be made smaller by folding the optical paths,making the apparent or effective projection lengths seem longer than theactual projection lengths.

[0076] Referring to FIG. 19, the display apparatus 200 includes an imageengine or projector 202. The image engine 202 may be similar to theprojection lens systems 10, 50, 150. The image engine 202 may also besimilar to the image engines described in the aforementioned U.S. patentapplication Ser. No. 08/730,818. The image engine 202 outputs imagelight 204 in response to input signals, for example, electronic, video,or other signals received from an antenna, cable, computer, orcontroller. The image light 204 (e.g., the image light 49 from theprojection lenses 12, 52 in FIGS. 1-4 or analogous image light from theprojection lens 152) reflects off a lower mirror or reflector 206 to ahigher mirror or reflector 208. The light 204 is then reflected by theupper mirror or reflector 208 and is directed to a screen 212, forexample, a diffusive screen or diffuser. The screen 212 (e.g., similarto the screen 36) scatters the image light as light 214, which a viewer215 can see as forming an image at the screen 212 of the displayapparatus 200.

[0077] Referring to FIG. 20, the display apparatus 250 is shown, whichincludes an image engine or projector 252, a signal splitter 254, aninput cable 256, a sound system 258, a screen apparatus 260, and a backmirror or reflector 262. The image engine 252 may be similar to theprojection lens systems 10, 50, 150 described above and those in theaforementioned U.S. patent application Ser. No. 08/730,818. The screenapparatus 260 includes a reflecting linear polarizer 264 and a screen268, which, depending on the specific design, may be layered, coated,bonded (e.g., with index matching adhesive), laminated (e.g., as oneelement), or otherwise applied together in the order shown in FIG. 20.The reflecting linear polarizer 264 and the screen 268 may be heldtogether with no air gap or with substantially no air gap.Alternatively, in other embodiments, the reflecting linear polarizer 264and the screen 268 may be held together in spaced apart relation.

[0078] The screen 268 (e.g., similar to the screen 36) may be adiffusive screen or a diffuser, and the reflecting linear polarizer 264may be constructed of MOF. Other polarizing reflector or wide-anglepolarizing reflector materials could also be used. The reflecting linearpolarizer 264 has the characteristic of preferentially reflecting lightof one linear polarization and preferentially transmitting light ofanother, linear but orthogonal, polarization, as discussed above.

[0079] The back reflector 262 includes a mirror or reflector 270 and anachromatic retarder 272 that, depending on the design, may be layered,coated, bonded (e.g., with index matching adhesive), adjacent orotherwise applied together in the order shown in FIG. 20. The backmirror or reflector 270 and the achromatic retarder 272 may be heldtogether in spaced apart relation or not be held spaced apart (i.e.,with substantially no air gaps). A suitable achromatic retarder 272 maybe designed to accommodate the spaced apart arrangement, as will beappreciated by those skilled in the art having the benefit of thepresent disclosure.

[0080] In operating the display apparatus 250, the image engine 252receives an electronic signal through the input cable 256 and providesthe signal to the signal splitter 254. The signal splitter 254 dividesthe signal into, for example, a video signal and an audio signal, andprovides these signals to the image engine 252 and the sound system 258,respectively. The image engine 252 converts the video signal intoprojected image light 274 (e.g., the image light 49). The electronicsignal received by the cable 256 may be any type of signal containingvideo information, such as a television signal received by an antenna orover cable lines, or a computer video signal received through a computervideo cable. The audio signal and the sound system are optional.

[0081] The image light 274 may be polarized in the image engine 252, forexample, by the operation of the reflecting linear polarizer 32, theimagers 38A, 38B, 38C, the pre-polarizer 86, and the clean-up element34, if present, as described above. A light source (not shown) in theimage engine 252 or other light source may be used to input linearlypolarized light initially into the image engine 252 in an illuminationsubsystem similar to those described above. The light would then beprocessed by the polarizer 32, the imagers 38A, 38B, 38C, and thepre-polarizer 86, the clean-up polarizer, if present, or as determinedby an external polarizer. The image light 274 may be polarized in thesecond polarization discussed above or have its polarization determinedby another polarizer that is employed external to the projection lens(not shown in FIG. 20) of the image engine 252. In a first instance, theimage light 274 output from the image engine 252 is polarized in thesecond polarization direction, for example. The light 274 is thenreflected by the reflecting linear polarizer 264 toward the backreflector 262. The reflected image light 274 passes through theachromatic retarder 272 a first time in one direction, is reflected bythe back mirror or reflector 270, and passes through the achromaticretarder 272 a second time, directed again toward the screen apparatus260. The achromatic retarder 272 is designed to have an opticalthickness of substantially one-quarter wave, such that the image light274 in the second polarization will undergo an effective half-wave(i.e., substantially 90 degrees) polarization shift or rotation ondouble pass through the achromatic retarder 272. Thus, the image light274, which is now directed toward the screen apparatus 260, willsubstantially be in the first polarization and will substantially passthrough the reflecting linear polarizer 264 and to the screen 268. Thescreen 268 scatters this light as image light 276. The viewer 215 canthen observe an image produced by the image light 276 at the screen 268of the screen apparatus 260, in similarity to the descriptions givenabove.

[0082] In all embodiments of the invention, diffusive viewing screens orbeaded screens may be used as the screens 36, 212, and 268. Beadedscreens capture stray imaging light, have a limited acceptance angle,and the stray light is absorbed in a black matrix. Diffusive screens, onthe other hand, scatter the stray light to improve homogeneity and/oruniformity in intensity across the viewing screen. The type of diffusivescreens discussed herein include bulk diffusive screens. Surfacediffusers, for example, ground glass and the like, could also be usedinstead of diffusive screens or beaded screens in accordance with otherembodiments of the invention.

[0083] The particular embodiments disclosed above are illustrative only,as the invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of theinvention. Accordingly, the protection sought herein is as set forth inthe claims below.

We claim:
 1. A projection lens system, comprising: an illuminationsubsystem comprising a light source, and a lightguide adapted to directillumination light from the light source; and a projection lens adaptedto receive the illumination light from the lightguide and to directimage light derived from the illumination light.
 2. The projection lenssystem of claim 1 , wherein the lightguide comprises a tapered lightpipe.
 3. The projection lens systems of claim 1 , wherein the lightguideis adapted to direct the illumination light by total internalreflection.
 4. The projection lens system of claim 1 , wherein thelightguide is disposed between the light source and the projection lens.5. The projection lens system of claim 1 , wherein the lightguidehomogenizes the illumination light.
 6. The projection lens system ofclaim 1 , wherein the lightguide provides the illumination light assubstantially telecentric light.
 7. The projection lens system of claim1 , further comprising a lens adapted to receive the illumination lightfrom the light source.
 8. The projection lens system of claim 7 ,wherein the lens is applied to the lightguide with an adhesive.
 9. Theprojection lens system of claim 7 , wherein a light filter is disposedbetween the lens and the lightguide.
 10. The projection lens system ofclaim 7 , wherein the lightguide is pressed between the lens and thelight source with no adhesive.
 11. The projection lens system of claim 7, wherein the lens is mounted in such a manner as to support thelightguide against the light source.
 12. The projection lens system ofclaim 1 , wherein the lightguide comprises a compound parabolicconcentrator.
 13. The projection lens system of claim 12 , furthercomprising a funnel adapted to accept the illumination light for entryinto the compound parabolic concentrator, wherein an angle defined bythe funnel determines the cone angle of the illumination light throughthe compound parabolic concentrator.
 14. The projection lens system ofclaim 12 , wherein the lightguide and the lens are mounted withoutadhesive.
 15. The projection lens system of claim 12 , wherein thelightguide is beveled at its contact with the lens.